The present invention relates to a zoom lens system that is suitable for use with a compact camera and which is subject to less constraints on back focus than zoom lens systems for use with single-lens reflex cameras. More particularly, the present invention relates to a compact and low-cost zoom lens system of a two-group type that has a zoom ratio of no more than about two but which has a wide coverage of angles, i.e., 36.degree.-37.degree. as half view angle, at the short focus end.
Conventional zoom lens systems for use with compact cameras are classified as two types, (A) a two-group type and (B) a three- or four-group type.
Compared with type (A), zoom lens systems of type (B) have the advantage of requiring relatively a small amount of lens movement but, on the other hand, they are not only large in size but also complex in construction. Because of these obvious differences from lens systems of a two-group type which are envisaged by the present invention, type (B) will not be described in detail hereinafter.
Compared to type (B), zoom lens systems of type (A) require a somewhat greater amount of lens movement but because of their simple lens configuration and mechanical structure, type (A) zoom lens systems have the advantage of ease in size reduction. Conventionally known zoom lens systems of a two-group type include version (A-1) that is described in Unexamined Published Japanese Patent Application Nos. Sho-56-128911, Sho-57-201213, Sho-60-48009, Sho-60-170816 and Sho-60-191216, version (A-2) that is described in Unexamined Published Japanese Patent Application Nos. Sho-62-90611, and Sho-64-57222, and version (A-3) that is described in Unexamined Published Japanese Patent Application Nos. Sho-62-113120 and Sho-62-264019.
In the zoom lens system of the present invention, a negative lens element is used as the first lens and Unexamined Published Japanese Patent Application No. Sho-63-276013 describes a similar telephoto zoom lens system of a two-group type in which a negative lens element is used as the first lens of the first group.
Version (A-1) has a small back focus and requires a large rear lens diameter, so it has had the problem that the overall size of the camera incorporating said lens system cannot be reduced. A further problem with this lens system is that internal reflections between the film plane and the last lens surface and other unwanted phenomena are highly likely to occur.
With a view to solving these problems, the assignee has proposed improved versions of a two-group type the back focus of which is comparatively large in consideration of its use with compact cameras. Such improved versions are (A-2) which is of a five-group-six-element composition and which is capable of a zoom ratio of 1.5-1.6 and (A-3) which is of a six-group-seven-element composition or seven-group-eight-element composition and which is capable of a zoom ratio on the order of 1.7-2.5. These versions range from a six-element composition capable of a zoom ratio of ca. 1.5-1.6 to an eight-element composition capable of a zoom ratio of at least 2. However, they provide a half view angle of only about 30.degree. at the short focus end and are chiefly intended to photograph scenery; in other words, they are short of satisfying the need to take pictures at wide angles with a compact camera. Further, those proposals have been unable to satisfy the need for providing a zoom lens system for use with a compact camera that is even more compact and less expensive.
The zoom lens system described in Unexamined Published Japanese Patent Application No. Sho-63-276013 uses a negative lens element as the first lens as in the present invention. However, this requires the use of lenses having a certain refractive index profile which are difficult to manufacture at low cost by the state-of-the art technology.
The present invention has been accomplished in order to solve the aforementioned problems of the prior art and its principal object is to provide a zoom lens system having a wide coverage of angles that is suitable for use with a compact camera and which is basically an improvement over the version (A-2) described in Unexamined Published Japanese Patent Application No. Sho-64-57222 in that the achievable zoom ratio and the lens composition are substantially the same as in (A-2) but which has a wide coverage of angles, i.e., 36.degree.-37.degree. as a half view angle at the short focus end, and which yet is not only smaller in size but also less expensive on account of the expensive use of a low-refractive index glass or plastic material.